IDEC-Y2B8 (IDEC-103) is a murine IgG1 kappa monoclonal antibody and IDEC- C2B8 (IDEC-102) is a chimeric IgG1 kappa monoclonal antibody, with mouse variable and human constant regions. Both antibodies recognize and show specificity for the CD20 antigen expressed on normal B cells and most malignant B-cell lymphomas. High-dose safety studies in cynomolgus monkeys have revealed no adverse clinical events with either antibody, and there were no significant abnormalities on laboratory tests or on histopathology. In vitro mechanism of action studies have demonstrated that IDEC-C2B8 binds human complement and lyses lymphoid B-cell lines. Additionally, it has significant activity in assays for antibody- dependent cellular cytotoxicity. As predicted, the biologic effect of these antibodies is manifested by B-cell depletion in peripheral blood, lymph nodes and bone marrow. In separate studies, several weeks after seven q.o.d. doses of IDEC-Y2B8, and 3 weeks after four weekly doses of IDEC-C2B8, a significant decrease of B cells in bone marrow was observed. Recovery of the B cells in the peripheral blood (to > 75% of baseline) usually occurred within 38 days and 60 days following the last dose of IDEC-Y2B8 and IDEC-C2B8, respectively.